Signal operated control for twochannel transmission systems



Feb. 20, 1951 ca. M. GIANNINI 2,542,921

SIGNAL OPERATED CONTROL FOR TWO-CHANNEL TRANSMISSION SYSTEMS Filed July2, 1945 4 Sheets-Sheet l 0X 90| )IHOMLEIN 2 SNIQNV IVQ r-l 0' t8 u. ldmib g cm I t CONTROLANETWOR INVENTOR.

GABRIEL M. GIANNINI ATTORNEYS Feb. 20, 1951 G. M. GlANNlN] SIGNALOPERATED CONTROL FOR TWO-CHANNEL TRANSMISSION SYSTEMS 4 Sheets-Sheet 2Filed July 2, 1945 m M mm N G E VM mL ..l. R B A 6 AT TORN EYS Feb. 20,1951 e. M. GlANNlNl 2,542,921

SIGNAL OPERATED CONTROL FOR TWO-CHANNEL TRANSMISSION SYSTEMS Filed July2, 1945 4 Sheets-Sheet 3 FIG?) INVEN TOR.

Q y GABIQTEL M. GlANNlNl ATTORNEYS Feb. 20, 1951 G. M. GIANNIN] SIGNALOPERATED CONTROL FOR TWO-CHANNEL TRANSMISSION SYSTEMS 4 Sheets-Sheet 4Filed July 2, 1945 INVENTOR. GABRIEL M. GIANNINI FIG. 5

FIG. 6

FIG. 4 334 ATTORNEYS Patented Feb. 20, 1951 SIGNAL OPERATED CONTROL FORTWO- CHANNEL TRANSMISSION SYSTEMS Gabriel M. Giannini, West Los Angeles,Calif., as-

s n rl by m sn a men to Aut mati Electric Laboratories, Inc., Chicago,Ill.,a cor.-

poration of Delaware Application July 2, 1945, Serial No. 602,710 I3,Claims. (Cl. 178-44) hybrid system or anti-side-tone impedance network is provided for preventing signal currents developed by operationof the transmitter 0r microphone from being transmitted to the receiveror loudspeaker.

In this type of arrangement one of the factors which determines theefficiency of the side-tone suppression is the impedance of the talkingcircuit established by Way of two connected subscriber lines. pedance isnot the same for any two different established connections, and,accordingly, the hybrid system of each substation is usually balanced toprovide maximum side-tone suppression efficiency for average lineconditions. In installations wherein amplification of incoming andoutgoing signal currents is required, as, for example, in executiveloudspeaking sets, the conditions of unbalance introduced in thesubstation circuit by the impedances of the lines over which anestablished connection extends may become intolerable. This isparticularly true in substation installations provided in an exchangearea Where the subscriber lines are of widely different lengths.minimize the singing which may result due to unbalance of the substationcircuit occasioned by unfavorable line conditions, signal controlledswitching means may be provided in the substation circuit forselectively blocking the channels when not in use. In one arrangement ofthis character the signal channels are completely blocked when not inuse. More particularly, during intervals when the transmitting means ofthe substation is being used to transmit outgoing signal currents, theincoming signal current channel is maintained completely inactive.Converse-1y, during those periods when signal currents are incoming tothe substation, the transmitting or outgoing signal current channel ismaintained completely inactive. An arrangement of this character, whilesatisfactory in operation, pre vents any interruption of a speech trainbeing transmitted in one direction in response to signal currentstransmitted in the opposite direc- This im- 1 In order completely toobviate or to tion. Inother words, conversation break-ins are positivelyprevented.v r It an obje t of th present invention to .110,- v d mprov dtelephone substation app us 03 he harac er des ri ed ere n o he t nsmision channels is n eier b v mai e at a no mal ga n and the opp s te nnis r e abl nar l v lo ked ut uf i y cti e to b w cm cahv respon iv o a aamma amount of sound induced currents, and wherein a control circuit isarrangedto'increase the gain of the partially blocked transmissionchannel to unblock the same and to decrease the normal gain .of theopposite channel to block the same in simple" and reliable manner. 1 ,Itis another object of the invention to pro? vide an improved transmissionsystem particularly suited for use in telephone substation circuits ofthe character described, wherein the con trol circuit is arranged in animproved manner ,to utilize electronic devices in order to render thecontrol circuit exceedingly fast in operation to "effect chan elblocking and unblocking operations without any substantial speechclipping.

It is a further object of the invention to provide an improvedtransmission system of the character described, wherein the screenelectrode of one of the amplifier tubes in each transmission channel isutilized as the control element for blocking and unblocking theassociated channel under the influence of a control circuit.

The invention, both as to its organization and method of operation,together with further objects and advantages thereof, will best beunderstood by reference to the specification taken in connection withthe accompanying drawings in which Figure 1 illustrates the substationcircuit arrangement of an improved system havingincorporated thereincertain of the features of the invention as briefly outlined above; Fig.2 is a perspective view of one formof a loudspeaker-microphonesubstation instrument which may be used at each of the stations of thesystem; Fig. 3 is a sectional view ofthe substation instrument of Fig. 2showing the usual instrumentalities ineluding the. loudspeaker, themicrophone, the ringer and the dial; and Figs. 4 to 8, inclusive,illustrate the details of the microphone and loud speaker assembliesprovided at each of thesubr stations of the system.

Referring now more particularly to Fig. l of the drawings, the improvedcontrol. circuit is illustrated as being incorporated in a loudspeakingsubstation circuit adapted to be connected to a telephone line too.terminating at the wind ings IOI and I02 of the hybrid system I03. Thetelephone line I may, for example, form a part of a conventionalautomatic or manual telephone system, in which case the distant endthereof will terminate in a line circuit located at the central bridsystem I03 and a balancing network I06 by means of the signaltransmission channe I01 and I08.

The outgoingsignal current channel I01 comprises a vacuum tube amplifierincluding the single amplifier tube I09 which is adapted to receivesignal currents from the microphone I04 1 and to deliver its output tothe hybrid systemj 03.

' 7 ing resistor H3 which is connected to the input electrodes of theamplifier tube I09. The output electrodes of the tube I00 are coupled tothe winding I2I ofthe hybrid system I03 by way ofthe conductor H0 andthe +3 terminal of a source of. anode current, not shown; Although theoutgoing channel I01 has been illustrated as having a single stage ofamplification, it should be understood that additional stages ofamplification or any suitable type, such as resistive, capacltlve orinductive coupled, may also be provided. The incoming signal currenttransmission channel I08 comprises a vacuum tube amplifier including thesingle amplifier tube' I22. More specifically, the incoming signalcurrent terminals of the hybrid system I03 are coupled to the inputelectrodes of the amplifier tube I22 through a coupling network whichincludes 'the winding I24 of the hybrid system I03 and an adjustablevoltage dividing resistor I25. The output electrodes of the tube I22 arecoupled to the primary winding I3I of the transformer I30 by way of theconductor I23 and the +3 terminal of the source of anode current, notshown. The secondary winding I32 of the transformer I30 is, in turn,connected to the voice coil of the loudspeaker I05. r

The control network I50, a indicated above, is provided for the purposeof increasing the gain of the signal current channel I08 anddecreasing'the gain of the signal current channel 101 when signalcurrent transmission isreceived over the telephone line I00. Brieflyconsidered, the control network I50 comprises an amplifier triode I39, afull-wave rectifier network I43 of the dry disc type, a twin triodeamplifier tube I46, and a pair of variable resistors I41 and I48. Morespecifically, the input conductor I33 of the control network I50 isconnected to one terminal of the adjustable resistor I25 included in theincoming channel I08 and to the input electrodes of the amplifier tubeI39 byway of the high pass filter comprising the condenser I34 and thevariable resistance I35, and the low pass filter comprising theresistance I36 and the condenser I31. The values of the resistances andthe condensers 4 forming the high and low pass filters are preferablysuch that frequencies between 700 and 1000 cycles are permitted to passand control the input electrodes of the tube I30. I38 shunting thecondenser I31 provides the necessary biasing potential for the inputelectrode of the tube I39. The output electrode of the tube I39 isconnected to one terminal of the primary winding I4I of the transformerI40 and the other terminal thereof is connected to the +B terminal ofthe source of anode potential, not

shown. The secondary winding I42 of the transformer I40 is connected tothe input electrodes of the twin triode amplifier tube I46 by way of therectifiernetwork I43 of the dry disc type and atime constant network,including the condenser I44 and the resistance I45 having its centraltap connected to a biasing battery included in the input circuits of thetube I40. The rectifiers comprising the network I43 may be of the copperoxide or the selenium dry disc type. The

output electrode I46a of the tube I46 is connected to the screen gridelectrode of the tube I09 and to the grounded condenser'l52, the lattercondenser providing filtering to prevent the impression of audio signalsthereon from the control network. The output electrode I46b of the tubeI46 is connected to the screen grid electrode of the tube I22 and to thegrounded condenser I5I, the condenser I5I providing filtering to preventthe impression of audio signals thereon from the control network. Theoutput electrodes Mile and I462) of the tube I46 are shunted by theseriesconnected variable resistors I41 and I48, the junction betweenthese resistors being connected to the +B terminal of the source ofanode potential, not'shown.

With no signal transmission over the line I00, the values of theresistors I41 and I48 are chosen so that the voltage drop through them,due to the anode currents of the tube I40 and the screen grid currentsof the tubes I09 and I22, is sufficient to bringthe operating screenvoltages of the tubes I09 and I22 to such a value that gains around theloop including the outgoing channel I01 and the incoming channel I08 areof such a value as to substantially eliminate the possibility'ofsinging. However, the values of the resistors I41 and I48 should also bechosen so that the outgoing channel I01 will normally have a maximumgain, consistant with stability of the circuit, to allow a normal amountof signal current transmission to pass therethrough, and so that theincoming channel I03 will nor-' mally operate at a gain level that iswell below the maximum gain;

With the system conditioned for operation, but inactive, no signalcurrents are transmitted through either of the two amplifier tubes I 09and I22. Therefore, no signal currents are transmitter through the bandpass filter network, the

' amplifier tube I39 and the rectifier network I43 The resistancecheaper:

transmitted over the celebrate line 100 through" thehybrid system- I03:sues signaiseuowevera are not reproduced in the incoming channel- IHowever, when a signal isreceivedover-theline- IIJU and the hybridsystern- I03-,- the amplifier tube I22 in the incomi'ngcha-nnel I08responds'in theusual manner tocontrol the"loudspeaker Iii-5:: However,signals reproducedby the -loudspeakei- I35 are not of sufiicientstrength to prdvide sat isfactory volume.

discriminating network; includingthecondensers I34 and; I3!andtheresistances' I35 and I36;-

This

and amplified by the amplifiertube" I39; 7 amplified signal isrectifiedby the rectifier net'- work I43 and appliedas a:direct'currentvoltagcacross the grids of the control tube I45: The grid" electrode associatedwith the output electrode Mfib is made more negative amine gridexectr'ode associated with the output electrode M611 is made morepositive? Consequently the anode current'of the output-electrode Idiiais increased;

thereby increasing the voltage" dropacross the resistor'ldlandthus-lowering the screen'bias on sistor Md and thus increasing thescreen volt;

age applied tothe' soreengrid of the tube 12-2. As the potential ofthescreen' grid of the tube I22 increases, the amplificationfactor of thistube is increased and thus causes unbloclging of the incoming channelIii-3. Accordingly, the out going channel It? iseffectively blockedtb-prevent the microphone led from responding to sig nals reproduced bytheloudspeaker IQB andthe incoming channel Hi3 iseffectivelyunblockedtopermit satisfactory amplification and reproduc tion by theloudspeaker Iil5- of signals received over the telephone line IUD;

From the above explanation it will be under stood that the increase insignal current gain through the incoming channel I D8 is accompae niedby a corresponding decreas'ein gain through the outgoing channel I'ill.The extent-to which the gains are changed is" dependent" upon thesetting of the adjustable tap on resistor I45. This tap is preferablyadjusted so that the increase in gain in channel- N38 is inversely prof"portional to the decrease" in gainin channel I131. Thus the stability ofthe-system is preserved. In this regard it will be noted' that thesystem includes a substantially closed signalcurrentcircuit whichcomprises the two channels I9 and I08 and the coupling path through thehybrid system IE3, and that if the two amplifiertubes I09 and IE2 areboth operated at the high gain settings required during signal currenttransmis sion, the total gain around the closed circuit, in-- cludingthe acoustical coupling path bet-ween the loudspeaker Hi5 and themicrophone Iii-4 may substantially exceed the electrical-and acousticallosses of the system. When this condition pea vails the system is highlyunstable and-an os' cilla'tory condition may be set-up therein even" A-portionof the sierra-l v'olt age, however, a is picked-up-through thefrequency and I48are restored to normal.

6. when signal currents of relatively" smali ani p1itlides aitransmittedtherethrough;- Due to the c6fitiol{-network- I5IJ; however,the total gain around' theclos'ed circuitof the system is maintainedbelow the predeterminedvalue at which the circuitis unstable byconcurrently increasing--the gain or the incoming channel I88 anddecreasing the gainof the outgoing channel Iill. Accordingly, thestability of' the'system is preseived even though signal currents ofsubstantial magnitude are received over the incoming channel I081 Eachtime signal current transmission receivedovei- 'the' incoiningchannelIE8 is terminated, the sign input to the control network I50 is alsoterminated; When this occurs the bias voltage acres-sane resistor I isrestored-to zero, whereby the space current paths through the outputeiec trddes -Idfid-a-nd l46b'of the-tube M5 and the volt age dropsacross the two biasing resistors I 41 The signal curreht gaiiithrough-theamplifier tube I09 is thus increased to normal and the signalcurrent gaintlii oiig'h the amplifier tube I22 is decreased to itsnormal partial blocking value. The rate at which thebias voltagesacrosstl e resistors I47 and I48 are restoredto normal is primarilydetermined by the time constant-of thenetwork comprising the resistor M5and the condenser I44 together with thevaiue ofthe grounded condensersI5I and I52. The-resistanceand capacitance values of these elements areproportioned toprovide the necessary hangover period at each signalcurrent brea'kofi point, in order to'prevent syllable clipping whichwould otherwise occur.

Referr ih'g now toF'igs 2, 3 and 4 of the drawings,- the substationinstrument'there illustrated comprises a casing 300 provided with foursubstantially perpendicular walls which converge and terminateinarconvexed', forwardly sloping microphone subc'asingsare filled withacoustical damping'mat'erial, such, for example, as felt.

More specifically considered, the main casing 300" comprises a slightlyinwardly sloping front wall portion- 301', a substantially perpendicularrear wall portion 302; and a pair of substantially perpendicular sidewall portions, the right side wetlltfifii heihg-illustrated in Fig. 2.The top wall p0rti'on3ii4 is somewhat convexed and is slightly slopedfrom the higher rear wall 362 toward the lower front wall 3i andconverges with all] the wall portions of the casing 386. A plurality ofapertures 305 and 306 are also provided in the casing Ball-andconstitute sound openings for the microphone IM and loudspeaker I05,respectively,

mounted within the main casing 380. A dialing mechanism 361 is securedin an opening in the top surfaceof the casing when the substationinstrinnent is connectedto an' automatic telephone" systehn'or in theevent the substation instrument is. connected to a manual telephonesystem a blank escutcheon plate may be substituted in place thereof.

For the purpose of permitting sound waves developed during the operationof the ringer 352, mounted within the casing 390, to be transmitted tothe surrounding air, a plurality of rectangularly shaped slots 309 areprovided in spaced-apart relation along the lower perpendicular sidewalls of the casing 300. A plurality of decorative strips 3IIl aresecured to the surface of the casing, which strips may be formed, forexample, of highly polished or plated I.

7 metal or of colored plastic material.

The casing 330 may be formed of an insulating A material, such, forexample, as Bakelite, or it may be formed of a metal and is preferablymoulded in one piece.

In forming the casing 303 the bottom thereof is left open in order tofacilitate the mounting of the various elements of the substationapparatus within the chamber defined by the above described wallstructure, a base'plate 3 being provided for closing the chamber definedby the walls of the casing. The casing 390 is also formed with acylindrical subcasing 3I2 which extends downwardly from' the top wall ofthe main casing 393 to a point substantially midway between the top andthe bottom thereof and 3I3 is provided for closing the cavity defined byV thecylindrical wall of the subcasing 3I2.

In order to prevent the diaphragm of the loudspeaker unit I95 from beingdamaged, a protective screen 3M of silk or fine mesh metal wire isprovided in the opening 336 and secured in place by cementing the sameto the peripheral edge portion of the opening or by merely clamping thesame in place under pressure. The rubber gasket 3I5 placed between thediaphragm supporting ring and the screen 353 placed over the opening 306in the casing 336 effectively prevents passage of air or sound wavesaround the edge of the loudspeaker diaphragm from the outside to theinside, or vice versa, of the chamber formed by ;of the loudspeaker andthe microphone diaphragms, and to prevent undesirable, resonanceeifectswithin the chamber formed by the subcasing 3I2, a suitable amountof acoustical insulating material such as, for example, a plurality V ofcompressible felt rings 3 i 6 are provided, which substantially surroundthe loudspeaker unit I05.

The loudspeaker unit I is mounted upon the cover plate 3 I3 by aplurality of screws 3 I I, one of which is shown in Fig. 3, and for thepurpose of preventing the cover plate 3 I 3 and any other solidstructure mechanically connected thereto from transmitting vibrationseither to or from the loudspeaker unit I 65, a mounting assembly of thecushion type is provided. More specifically, the annular recess providedin the soft rubber grommet 3I8a snugly engages the adjacent-surfaces ofthe opening formed in the cover plate 3 I3, and a mounting screw 3 I 1having ametal washer 3I9-anda locking washer 32I extends through acentral opening provided in the grommet 3I8a and through the centralopening of the soft rubber grommet spacer 3I8b, and is threaded into a 5tapped hole provided in the loudspeaker unit I05. Preferably three suchcushion type mountings are provided to resiliently secure theloudspeaker unit upon the cover plate 3I3.

For the purpose of securing the cover plate 3I3 1 tov the subcasing 3 I2, a plurality of equally spaced tapped holes are provided into whichthescrews' 322a, having locking washers 322b, are threaded after theyhave been inserted in corresponding holes provided in the cover plate 3I3. A soft rubber gasket 323 is placed between the cover plate 3I3' andthe bottom surface of the subcasing 3I2,'

' whereby the screws 322a may be utilized to draw the loudspeaker unitI05 upwardly to securely clamp the flexible soft rubber gasket 3 I 5between the diaphragm supporting ring of the loudspeaker unit I05 andthe under surface adjacent the sound opening 306 in the main casing 300.The screws 322a are also efiective to securely clamp the soft rubbergasket 323 between the cover plate 3 I3 and the bottom surface of thesubcasing 3I2.

The soft rubber gasket 323 functions as a seal beand prevents, incombination with the acoustical insulating material 3I6, sound wavesemanating from the rear surface of the loudspeaker diaphragm frompassing to'the front surface thereof and from passing to the microphone.diaphragm. The densityof the acoustical material 3 I 6 surrounding theloudspeaker unit 105 may be altered in order to vary the frequencyresponse of the loudspeaker diaphragm. Thus the frequency responsecharacteristics of the loudspeaker areenhan'ced and troublesomeacoustical coupling between the sound waves created by the 40 rearsurface of the loudspeaker diaphragm and the microphone is prevented.

Since the in and out wires (not shown) forming the voice coil of theloudspeaker unit I05 are somewhat fragile, they are soldered toterminals secured to the diaphragm support. A pair of heavier strandedlead-in Wires are also soldered to the terminals, as will be descripedhereinafter, in order to provide somewhat flexible electricalconnections between the voice coil wires and the to exterior of thesubcasing 3I2. In order to facili- "tate electrical connection betweenthe incoming amplifier I68 and. the stranded lead-in wire extending tothe voice coil terminals, a terminal block 324 of insulating materialhas been prosa vided whereby the above-mentioned stranded lead-in wiresmay be secured'thereto for electrical connection with the conductorsextending to the incoming channel amplifier I08. More specifically, theterminal block 324 comprises a oq plurality of terminals 324a, securedthereto in any "suitable manner, which are adapted to accommodate theterminal screws324b. The terminal block 324 is held in spaced relationupon the cover plate 3| 3 by the metal spacers 325 and the screws o5,,322b, which are somewhat longer than the cover plate securing screws322a, and are threaded into tapped holes in the bottom surface of thesubcasing 3I2 to'securethe terminal block and the cover plate inposition. The above-mentioned 70,, insulated wires (not shown) areinserted through a central opening provided in the soft rubber grommet326, which snugly engages the adjacent surfaces of the opening formed inthe cover plate 3 I3, and are directly secured to the terminals 324a .7hum ns of th s rews, 324p. In order to com-,'. a

tween the cover plate 3I3 and the subcasing 3I2 amiw 9; pletel y sealthecavity fomied' by thesubcasi-n'g 312 1 and to revent" sound waves fromassing through the central opening in the grommet'326, said opening maybe 'se'aled with" sealing wax or thelike;

From the foregoing description it will heunderstood that the loudspeakerunit lflii is mounted in a subcasing 3l2 whichisa p'art of and integralwith the main casing 300. Furthermore, it will be understoodthatalthou'gh sound Waves created at the rear surface of theloudspeakendiaphragm are completely isolated from-sound waves created bythe front surface thereof andviee' versa, the

loudspeaker unit may be removed from and 3 replaced in the subcasing 3 I2 in order that it maybe readily adjusted and repaired.

It will be seen that the microphone unit [04 is mounted in a subca'singwhichis' deta'chably mounted in the main casing'300l More specifically,a protective screen 330 of "silliorfinemesh metal wire is "provided" inthe opening-305 and secured in place by cementingthe same to-theperipheral edge portion oftheppening. Arubber gasket 33% is placedbetweenthe shallow cupshaped-member 32801? the microphone subcasingandthe protectivescreen330' in the opening 305 of the maincasingtofurther'clarnp the periphera edge portion;- of I the" screen surroundingthe opening 305 The screen 330' and the rubber gasket'33'l may besecuredin place by cementing the same to the peripheral edgeportion-around the opening 305, orthey may heme-rely clamped in place bypressure appliedin mounting the subcasing for the microph'one'unit? The-sub'c'asing 'for the microphone lll' icomprises the fiont cupshapedmember 3'28 andthe rear cup shaped member 327; the latter cup sh'apedmember being the portionto which themicroplion'e'l'll' l and theacoustical material is mounted before the subcasing is inserted andsecuredin place'in the main casilng 300. A detailed description of thevarious e ements mounted Within the subcasing comprising the members'32Tand 32'8- will be given subsequently irrc'onnectiori with Figs: 4tot} inelusive.

Referring again to Fig. 3; it will be-noted that in" order to mountthecasing 32! in the main casing 3 a plurality of L sliaped' mountingbrackets 329 are secured'to therear'cup-shaped casing member 321in'spacedrelationaroundthe outside surface thereof. TheL-shaped bracketsare each prov'idedwith an-openinginto which a soft rubber'grommet 338 isinsertedso that the annular recess of the grommet snugly engages the a'da'cent surfaces of' the said opening. A mounting screw 340. providedwith a metal washer 339a and a locking Washer 33%; extends through thecentral opening inthe grommet 3313 andis threaded intoa' tapped" holeprovided in an ass'ociate'd"leg-shaped member" 331 in the casing 360.Preiera'bly three such cushion type mountings are provided to-s'ecurethesubcasing 3 2 l'-3Z'ii= and" the microphone unit I M in properrelationship with'the opening 305'. Furthermore the cushion typemounting. referred" to above, in combination with thesoft rubber gasket33l engaging the peripheral edge of the opening 305, is utilized topermit the microphonemnit and .its associated casingto' be tightly"clamped in posi tion and therebyprevent any sound waves' from reachingthe-'- rear surface of" the" microphone diaphragm.

The individualmicroph'one' unit I04 is identical with the" loudspeakerunit I; withv the: except tion that -the diaphragnrfor the microphone [M.isiprovided with lan'zoil I silk firing cemented tolthe peripheryf ofthe: moulded fiber cone portion whereas: theitdiaphragmdon'theuloudspeaker. I05 is a:single unitarystructure of moulded fiber, asissillustrated iniFligsz-b and: 6, respectively. Referrin'gi nowitorFig.4;-.aidescription:wi1l be given ofithervariousestructuralelements'forming the microphonevunit I041. It should 3 be understood,however that .thissdescription also applies 'tOT 'bhG loudspeaker unit:with the exceptions noted above; Inzthisw figure; all the parts formingthe microphone": unit I 04". areshown "inexplodedirelationshipitmeach"other, togetherzwith the'various e'lementsconstituting: the subcasing assembly utilizedtini mounting themicrophone within": the main'fcasing'300i Theimicrophone assemblycomprises? a Selle-contained" magnetic. circuit which includesa:permanently magnetized 'annular' ring 4mhformediofAlnico';a-*bottom.plate 40 I, a center polelpiecer 4-02; andia tcp plate-403:The parts 40F; 402 and? 403: are: preferably formed of Alleghenyelectriccm'etal iori otherhighly. permeable". magnetic material. The:center pole: piece 402 is providediwithrw lower portion 402d ofxreduoe'ddiameter;iwhichiextendsrwithin-a centrally disposed opening provided inthe. bottom plate 40! i Al sweat" connections between the side wallsofthisiopenin'g and the sides of the "extended porition :402a!may-berivetedrover on the bottom surfa'ce of the =plate40 I? and thus rigidlysecure the center DIZflGHQiB'CB :4'02 to tliesbottom plate 40li Atitsfuppenendathecenten pole piece-4 02 :isprovided with: at. portion a402m of reduced diameter, which extendsiwithin a-centrallydisposedopening 403a formedlinit'lie toprp'late 403:. A's best shown inFigifi of thedrawings the diameterf of the opening 403:; r is= slightlylarger than the 'dia'meten of theiippenportioniwib.of thepole=piece:4'02. The upperr end of the center pole: piece isconcentrioallydisposed withinthenpening-GllSa whereby an annular"air-gap is zformed between: the adjacent side wallsof the 'openingiandthe center pole piece; The magneticcircuit assembly is completed by-'means of three: clamping screws 404 which extend-through" openingsprovided inthe bottom plate 4M and thertop plate-403, and'arethreadedaintoxtapped holes driIIedPin the bottom plater40 B ofwthediaphragm support basket as.- sembly:- Thesevscrewscserve:rigidly toclamp the annularperm'anent magnet to D-between the? topandibottom-platesA-M and 4M and to secure the diaphragm". supportwassemblyin proper" align;- ment withrthe centen polelpiece 402. i

The: moving: system; of" the microphone comprises'l a domes-shapeddiaphragm 405. which is formed of moulded fiber or the like and iscarfr-ied by the diaphragm support basket assemblywhicliicomprisesthe:bottomplate 400- and adiaphragmi supporting: ring407 having three inwardly extending; L shape'd brackets 408- which areweldedmo theupper surface ofthe-plate 406. The: moving: system alsoincludes two paper washersz4-Hl? and 4H betweenwhich theperipheralredgeiof the. diaphragm 405 is clamped. The character. of:the:- particular. diaphragm utilized dependss uponiwheth'en the deviceisto icensed as: a loudspeaken on as: a microphone. If the device is: tobe: used: as a loudspeaker thedialplira-gmi is of; onetpiece mouldedconstruction including an inverted dome-shaped; portion. 405, a;concentric annulari corrugated portion, and: an outer: fiat? annularring: portion.- 405a which: extends betweeni. and: is cemented. to: thepaper washersr4l Grand. 44 2h inithemanner illustrateddn Fig; 6;lithe-device? isito be? used as a micro"- phone the diaphragm'construction jillustrated in Fig. of the drawings is used. In thisconstruction the inverted dome-shaped portion405 of the diaphragm isformed of moulded fiber, the

periphery thereof being cemented to an annular'ring 405D of oil silkwhich is disposedbetween and cemented to the paper washers M0 and. M I;The purpose of the latter arrangement is. to per- :mit substantiallyfree vibration of the diaphragm element 405, particularly at the lowfrequencies of the operating frequency range, whereby the response ofthe device at these freand, accordingly, the response thereof,particularly at low frequencies, is'substantially less than when thediaphragm having the peripheral oil silk'ring' is used. By virtue ofthis increased 'stiifness, low frequency noise components'of soundareprevented from producing any substantial response of the device.

A moving voice coil M3 is cemented to the lower inverted dome-shapedportion 405' of the diaphragm and is concentrically disposed within theair gap defined by the upper portion 4021) of the center pole piece 402and the opening 403a in the top plate 403. The ends of the fine wirevoice coilmay be connected to flexible stranded lead-in wires in anydesired manner, although preferably the connections are-made bysoldering the ends of the voice coil wires to terminals secured to theL-shaped brackets 408 of the diaphragm support basket assembly, as isillustrated in the enlarged views shown in Figs. '1 and 8. Morespecifically, the L'-shaped bracket 408 is provided with an openingtherein'intowhich a pair of insulating washers 408a are inserted havingenlarged shoulder portions adjacent the surfaces surrounding theperiphery of the said opening. The insulating washers 408a are providedwith a central opening into which a brass eyelet 4081) is inserted,which has a shoulder portion formed at one end and is riveted over atthe opposite end, thereby to 'maintain the eyelet 4081) and theinsulating washers 408a in assembled relationship with the L-shapedbracket 408. The ends of the heavier stranded lead-in wires (not shown)are also soldered to the abovementioned eyelet terminals 4081) and theopposite ends thereof are connected to the terminal block 342, in thesame manner as has been described hereinbefore in connection with theloudspeaker I05, for electrical connection with the conductors extendingto the outgoing channel amplifier I01. In order to preserve the correctlateral and axial s acing of the moving coil M3 in the air gap of themagnetic circuit, and to provide an additional support forthe'diaphra'gm 405, a string supporting arrangement is provided.

This arrangement comprises three connected strings M400, 4 I41) andwhich extend through equiangularly spaced openings cut through the lowerportion of the diaphragm member 405 and like and it will be understood"that "by a'pproplurality of assembly screws 419.

priately tensioning the three strings the required support for themoving coil 3 may be obtained. v e

In order to seal the annular cavity, formed between the spaced-apartannularpermanent magnet; 400 and the center pole piece 402-, from the'acoustical'cavity adjacent the lower side of the diaphragm 405, therebyto enhance the damping of the diaphragm and thus producea more uniformresponse thereof over the entire operating frequency range, a sealingring 4 I8 is provided which snugly encloses the upper end portion4ll2bof the center pole piece 402, and is clamped to the underside of the topplate 403 by meansof a Thi ring s preferably formedof brass or othernon-magnetic material and to further insure the complete sealing of thespace between the sealing ring M8 and the-portion QGZb-of thecenter-pole piece 402 a rubber gasket Him is cemented at thejuncture ofthe sealing ring H8 and the end portion 4021). As pointed out above,each loudspeaker and microphone unit, as constructed in the manner justdescribed, ispreferably surrounded by acoustical insulating material,such as a plurality of felt rings, to prevent the transmission of soundwaves to or from the rear side of the diaphragm of the unit. In thearrangement illustrated in Fig. 4, which is an exploded view of themicrophone and casing therefor shown in the righthand portion of Fig; 3,the cup-shaped casing member 321- constitutes the subcasing for themicrophone assembly; r e The microphone unit l04'is mounted in thecupshaped casing 321 by a plurality of screws 334, one of-Which is shownin Fig. 3, and for the purpose of preventing .the'casing 321 andanyother solid structure mechanically connected thereto" from transmittingvibrations to the microphone unit, a

mounting assembly of the cushion type is provided. More specifically,the annular recess provided in the soft rubber grommet 334a snuglyengages the adjacent surfaces of the opening formed in'the bottomsurface of the casing 321, and a mounting screw 334 having a metalwasher 335 and a locking washer 336 extends'through a central openingprovided in the grommet 334a and through the central opening provided ina rubber grommet spacer 33412 and is threaded into a tapped holeprovided in the bottom'plate 40| of the" microphone unit. Preferablythree such cushion type mountings are provided to secure the microphoneunit I24 to the casing 321.

- In order to prevent acoustical coupling between the rear surface ofthe microphone diaphragm and the surrounding air, and to preventundesirable resonance effects within the cavity formed by the casing321, a suitable amount of acoustical insulating material is provided inthe cavity and substantially surrounds the microphone unit. Morespecifically, in securing the microphone unit 7 104 acompressible feltring 333a is placed bethe microphone unit-i04 in thecasing 321, a softrubber gasket 332 may becemented to the paper ring 4H which secures theperipheral edge of the microphone diaphragm on the supporting ring 501in order to provide a cushion seal between the edge of the diaphragm andthe cup-shaped casingrcover 3ll8. ,The open end of the casing cover 1 3328='is sli ghtly larger tiran thenpen nactments iiig 32:1 and providesclose flttingl cover for compressing the; soft rubber gasket 3 32- and'the felt ringssssuand 33%;- Themic'rophone unit 104 audits individualsutcasmg' now' be mounted iri the previously described manner tocooperate with the opening 305 provided in" the maineasing 300.-

A tei miiial' block 3E2 oi insula'ting material has been provided inorder toteiiriinate tlie' incoming' eleeti ical conductors extendingbetween the outgoing?amplifier I01 and the voice coil of the microphcrleun'it' l E34. These incoming conductors provide the necessary electricalconnection between" the o'utgoing channel amplifier F07 and the voice:coil leads of the microphone diaphragm:- More specifically,theteiinin'al bleck 342- comprises a' plurality of terminals 342asecured thereto any-suitabldmanner, which are adapted to accommodate theterminal screws 3 2b.- The incoming conductors may" be detachablyconnected to the" terminals by means of the terminal screws 34%, and thestranded lead-in wires exterid-ing to the diaphragm' voice coilleads,

which aTresolde'r'ed to' the terminals in thediaphlagin" supportassembly, may be" electrically c'o'nnected by"solderingthstiandedilead-in wires to the terminals 342d. The latter leads arethreaded thrbii'gh the eenu-aiopening provided in the sot-t riibber'grommet 5. M, which snugly engages the a'dja'cent {surfacesoftheop'ening formed in -the"botteni of 'the casing 32 1, and aresoldered to the terminals 3K2r as nieiitioned above. In order tocompletely seal the cavity formed by the casingtfl andto prevent soundwaves from passing through the central opening. in the grommet 344, thecentral openiiigin'the grommet may be sealed with sea'ling wax orthelike; In order to provide a suitable mounting forthe' terminal blockupon the surface of the casing 32 a pair of threaded spacer lugstcl areamxe'd to the rear surface o'f the casing in any suitable manner, suchas, for example, by welding orsoldering. The screws iits provided withthe usuallockingw'ashers may be" inserted throughappropriateholesprovided in the terminal strip-34'2" and threaded into' the tapped holesin the-spacers 3 31, thereby torigidly secure the terminal strip 342 inspaced relation upon the rear ofithecasiiig 3215 As shown, the topsurface 3M of the casing'3llll is' provided with an opening intc whichthe dial mechanism 391 is inserted so'that a portion of the bottomsurface of the dial 301 rests upon the shoulder portion 345 which isslightly smaller in diameter than the diameter of the dialing mechanismitself, The dialing mechanism is secured in the above opening by meansof a plurality of mounting screws 346a and associated locking washers3461) (one of which is shown in Fig. 3). As diagrammatically indicatedby the dotted line 341, a portion of the operating mechanism of thedialing mechanism projects through the opening provided in the topsurface of the casing 30!], but it is so situated that it does notinterfere with the removal of the microphone casing 321 from the maincasing 308.

Since the substation instrument is to be utilized in connection with atelephone system, a portion of the telephone apparatus utilized insignaling a called substation may be mounted upon the base plate 3| I.More particularly, this apparatus comprises a condenser 355 and a ringer352 mounted upon the base plate 3!! in the same manner as is describedin detail in the Obergfell Patent No. 2,272,474, granted February 10,1942.

Briefly considered; the mounting; arrangement for the condenser 355comprises abracket member 3542 secured to the side of I the condenserhousing; has a U-shaped' configuration" and is indiifit'ed upon thecover pla'te 3'! l by means of screws extenuin through the base plat'eand threaded into tapped openings provided in the bracket 3 54, theheadsof the screws being effectively countersunk in raised wells formed inthe base plate 3| I l in the manner illustrated:- The ringer SSZcomprises a field structure including a polarizing permanentmagnet 35!and a' pair of windings, one of which is indica'ted at 353. Alsoembodied in the ringer are a pair of bells, one of which is shown,carried by a bracket 35'! anol a striker assemblyjnot shown, ofconventional-arrangement. For the purpose of preventing the base plate3| l and any other solid structure mechanically connected thereto fromexerting any damping influence'upon the bells of the'ringer 352' duringthe'operation of the ringer, and also for preventing vibrations createdby the ringer from being. transmitted through the solid structure of theinstrument to either the microphone we or the loudspeaker H35, a=mountingassembly of thecushion type is used for supporting the ringerupont'he base'plate 3| I. This mounting assembly comprises a pluralityof soft rubber grommetsiand the usual mounting screws which secure theringer assembly and the associated grommets to the'base-member 3| I inthe manner described in the 'above-mentioned Obergfell patent. For thepurpose-of permitting sound waves developed during the operation of theringer 352 to be transmittedthroughtheoutside casing 3B0, a plurality ofrectangularly shaped slots 339 are provided in evenly spacedapartrelation around the lower perpendicular side walls of the casing 3%. Inorder to prevent foreign material and dust frombeing lodged within thecasing 355, the slots 30!) may be covered by a fine mesh wire screen or'the like which is secured to the interior surface of thecasing member350.

For the purpose of detachably securing the'base plate 3'! I to thecasing 308a plurality of mounting lugs 35lla and 35ilb are moulded orcast as an integral part of the casingand project in- Wardly from theperpendicular rear and side walls thereof. A--third lug, not shown, isformed in-the opposite-side wallof the casing 300. Each of the mountinglugs is drilled and tapped to receive a'threaded'portion of the mountingscrews, such as screws 349a and 34%. The screws are locked to the baseplate 3! l and the heads thereof are eifectively countersunk in raisedwells provided in the base plate in the manner illustrated. Screws 3 19aand 35th are threaded into tapped holes in the mounting lugs 3563a and35% respectively and serve, together with the not shown third screw andassociated mounting lug to clamp the rim of the base plate 3! againstthe receiving ridge formed in the lower edge of the casing 39! For thepurpose of preventing the base plate from contacting the surface of theobject upon which it is supported, a flexible cushioning element 348formed of soft rubber is stretched around the rim of the base plate 3!i, so that a portion 348a thereof extends inwardly around the peripheralportion of the base plate and a portion E i-8b thereof is clampedbetween the base plate and the bottom ridge portion of the casing 300when the base plate is mounted upon the casing by the screws 349a and3491).

The substation instrument described above is disclosed and claimed inthe copending divisional application of Gabriel M. Giannini, Serial No.

channels, two amplifier tubes respectively included in said channels,two screen electrodes re spectively included in said tubes, a controlcircuit including two output electrodes respectively connected to tworesistors and a common source of current connected to said resistors andtwo biasing conductors respectively connected between said screenelectrodes and said resistors, means 'for normally establishingtwocontrol currents respectively traversing said output electrodes andsaid resistors and commonly traversing said common source of current inorder normally to establish two biasing potentials applied to saidscreen electrodes so that a maximum signal gain is normally establishedthrough one of said channels and a below maximum signal gain is normallyestablished through the other of said channels, and means responsive tosignal currents traversing said other channel for varying in oppositesenses said control currents traversing said output electrodes and saidresistors in order'to vary in opposite senses said two biasingpotentials normally applied to said screen electrodes so that the signalgains through said channels are var'i'ed in opposite senses. I

2. A'transmi ssion system comprising two signal channels, two amplifiertubes respectively includ ed in said channels, two screen electrodesrespec tively included in said tubes, a control tube includingtwo'output electrodes respectively connected'to said screen electrodes,means for normally establishing two space currents respectivelytraversing said output electrodes in order normally to establish twobiasing potentials respectively applied to said screen electrodes sothat a maximum signal gain is normally established through one of saidchannels and a below maximum signal gain is normally established throughthe other of said channels, means including a rec tifier responsive tosignal currents traversing said one channel for producing a rectifiedcurrent, means for developing two control potentials of opposite sensesfrom said rectified current and for impressing them to vary in oppositesenses said normal space currents traversing said outputelectrodes inorder to vary in opposite senses said normal biasing potentials appliedto said screen electrodes so that the signal gains through said channelsare varied in opposite senses.

3. A transmission system comprising two signal channels, two amplifiertubes respectively included in said channels, two screen electrodesrespectively included in said tubes, 'a control tube including twooutput electrodes respectively connected to said screen electrodes,means for normally establishingtwo space currents respectivelytraversing said output electrodes in order normally to establish twobiasin potentials respectively applied to said screen electrodes so thata maximum signal gain is normally established through one of saidchannels and a blocking signal-gain is established in the other of'saidchannels, means including a band pass filter connected to said otherchannel for only passing signal currents in a predetermined frequencyrange traversing said other channel, means including a rectifierconnected to said filter and operated responsive to signal currentspassed thereby for producing a rectified current, and means fordeveloping two control potentials of opposite senses from said rectifiedcurrent and for impressing them to vary in opposite senses said spacecurrents traversing said output electrodes in order to vary in oppositesenses said normal biasing potentials applied to said screen electrodesso that a blocking signal gain is established in said one channel and amaximum signalgain is established through said other channel.

GABRIEL M. GIANNINI.

7 REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Baird Feb. 27,1945

